Thermal energy may be used in various ways for providing mechanical work. One machine for achieving this is the Stirling engine, i.e. a heat engine that uses different temperatures of a working fluid for converting heat energy to mechanical work.
Different types of Stirling engines have been suggested over the years, of which one, commonly denoted as the beta type, has a cylinder enclosing a fixed amount of air. One end of the cylinder is a hot side, while the opposite end is a cold side. A power piston is moveable within the cylinder due to compression and expansion of the air caused by a temperature change. For this a displacer, in synchronous movement with the power piston, is arranged within the cylinder which distributes the majority of the enclosed air either on the hot side or the cold side by a linear movement. Hence, the frequency of the reciprocal movement of the power piston may be controlled by adjusting the temperatures of the hot and cold sides, respectively.
Although the above-described engine may be utilized for a number of applications, it is difficult to control the power piston with a rapid response due to delay in heating the working fluid. In order to solve this problem, it has been suggested to disconnect the displacer from the power piston, such that the frequency of the power piston may be directly controlled by setting the frequency of the displacer movement.
A yet further improved machine is described in US2003000210. Here, a rotating displacer has a peripheral cutout for moving working fluid through a heat zone and a cold zone, each zone extending 180° along the periphery of the enclosing cylinder. Pressure variations inside the peripheral cutout are converted to mechanical work as the displacer rotates, and a regenerator is further provided for decreasing thermal energy losses. The proposed machine has a significant disadvantage. For the power to increase, the peripheral cutout needs to accommodate a relatively large volume of working fluid. Hence, the peripheral cutout must either have an increased height or a large angular extension. On the other hand, if the angular extension is made too large, there will be simultaneous cooling and heating of the working fluid when the peripheral cutout is partly on the hot side, and partly on the cold side as the displacer rotates. Hence, it will not be possible to increase power output without efficiency loss, and vice versa.